In an all-wheel drive motor vehicle with a second drive axle (usually the rear axle) driven via a slip-controlled coupling, the problem exists that the rear wheels tend to over-brake and thus lose ground adhesion, leading to skidding of the vehicle. This happens because of the connection to the front wheels. The condition is especially apparent when the vehicle employs an ABS system, and in cases of lock-up braking.
For this reason the additional utilization of a dual action free-wheeling coupling (sometimes referred to as an overrunning clutch) is customary so that a controllable separation coupling does not have to be used, and yet all-wheel drive in reverse travel is possible. For this reason dual-action clamping element couplings have been proposed, in which differential rotational speed sensing elements ensure coupling in both directions of torque flow and where centrifugal elements prevent a torque flow reversal at high speed (e.g., the German patent applications DE 42 01 375, 42 02 152 and 42 25 202 by the assignee herein). These solutions have the disadvantage, however, that when the circumferential speeds of the front and rear wheels differ because of tire wear or due to layout necessities, stress within the vehicle occurs at low speed and low load causing not only performance losses but also causing the clamping elements to be held in the clamped position. For this reason other solutions to this problem have been investigated.
The instant invention therefore relates to a rotationally dependent free-wheeling coupling comprising the following elements: a first ring which is the driving element in towing mode; a second ring which is the driven element in towing mode; clamping elements which act between the first and the second rings in both torque flow directions; a cage which holds the clamping elements; and a friction element actively connecting the cage to the second ring. (In this description, the "towing mode" refers to the situation wherein the engine drives the vehicle and torque flows from the engine to the axle, whether the vehicle is in forward or reverse travel. "Thrust mode" is the situation wherein the engine brakes the vehicle and torque flows from the axle to the engine.)
Such a free-wheeling coupling with clamping or wedging rollers acting in both directions for the transmission of drive forces to a second drive axle of a vehicle (here the front axle) is known from DE-A 27 40 638 (U.S. Pat. No. 4,124,085). A frictional connection exists between the ring pertaining to the second drive axle and a clamping roller cage. When overrunning occurs, a latch mechanism controlled via a second frictional connection to the fixed housing prevents the clamping roller cage from moving into the position in which towing or thrust torque flows from the second drive axle to the drive mechanism of the vehicle or to the first drive axle.
If reliable switching is desired in the free-wheeling coupling described therein, it is necessary to use a frictional element sensing the absolute rotational speed which has the disadvantages of relatively high power loss and a tendency to wear. Even so, reliable switching at extreme accelerations, e.g., with lock-up braking, is not ensured because of the high inertial forces acting on the cage. Under extreme conditions of torque change, "breakthrough," a situation wherein the clamping elements disengage from the ramps associated with one torque flow direction and overshoot to the ramps for the opposite torque flow direction, can occur. The free-wheeling operation may thus be bridged even in towing mode. This can result in locking of the wheels of the second drive axle, a loss of braking of stability, and even destruction of the free wheeling device under certain circumstances.
Another dual action free-wheeling coupling of a similar type was proposed in the as yet unpublished German patent application DE-43 11 288 (corresponding to U.S. application Ser. No. 08/222,802, filed Apr. 5, 1994) of the presented assignee. In the device described therein, completely friction and wear-free operation and protection against breakthrough when the direction of torque flow changes is ensured in all continuous operation states due to the interaction of a latch with the cage under the influence of a retaining spring and a slipping spring. However, there are still situations even with this free-wheeling coupling when faultless operation is not ensured. Even if such situations occur rarely, it is necessary to correct them in view of the safety standards applied today in the construction of motor vehicles.
Such a situation arises, for example, as follows: When the vehicle climbs a slope in reverse and torque is thereby being transmitted to the rear axle, the free-wheeling device assumes a configuration that is not an accessible configuration for forward travel in the thrust mode. Thus, when the driver stops the vehicle, shifts to a forward speed and lets the vehicle roll without operating the accelerator, the free-wheeling device cannot shift into the proper configuration. It is therefore not certain that the free-wheeling coupling will have zero torque in case of subsequent ABS braking.
It is therefore the object of the invention, while avoiding the disadvantages of the two designs described above, to provide a free-wheeling coupling sensitive to the direction of rotation which has at its disposal sufficient switching force, with minimal friction losses, in all continuous operation states to access the proper configurations so as to avoid such problems in exceptional situations.